Separated reactants processing of high Tc superconductors

ABSTRACT

A method for forming a conductor element comprising a Tl or Hg-based high temperature superconductor (HTSC) material, comprises providing at least one first precursor material within an outer sheath for the conductor element; providing at least one second precursor material within the conductor sheath and separated from the first precursor material(s) by a barrier layer formed from a Noble metal for example between the first and second precursor materials; and heating the conductor sheath containing the precursors to a temperature at which the barrier layer melts to allow the precursor materials to mix and react, or to a temperature at which one of the precursor material(s) diffuses through the barrier layer sufficiently allow the precursor materials to mix and react, to form the Tl or Hg-HTSC material within the outer conductor sheath.

FIELD OF INVENTION

The present invention comprises a method for forming cuprate. Tl orHg-based high temperature superconducting wires, tapes and otherconducting elements.

BACKGROUND

Many High-T_(c) Superconducting Cuprates (HTSC) are known to havesuperconducting transition temperatures, T_(c) exceeding the temperatureat which liquid nitrogen boils, 77 K. As such they have a potentiallylarge number of applications ranging from power generation,distribution, transformation and control, to high-field magnets, motors,body scanners, telecommunication and electronics. T_(c) values may be ofthe order of 92 K for example for YBa₂ Cu₃ O₇₋δ, 109 K for example forBi₂ Sr₂ Ca₂ Cu₃ O₁₀, 120K for example for TlBa₂ Ca₂ Cu₃ O₁₀ or as highas 134 K for HgBa₂ Ca₂ Cu₃ O₁₀. For many of these applications suchT_(c) values alone do not guarantee the utility of these HTSC at 77K orhigher temperatures. Often these applications require large criticalcurrents in the HTSC and this is not achieved unless the grains of theHTSC are crystallographically aligned, otherwise known as textured, andwell sintered together. This is commonly achieved in thin-films whereinthe HTSC material is deposited on a substrate in such a way as to obtaincrystallographic alignment of the material. However, thin films, whilesupporting very high critical current densities, J_(c), do not carry avery high absolute critical current, I_(c), because they are so thin.Superconducting wires which use bulk superconducting material can inprinciple support much higher I_(c) values provided they can be texturedto achieve high J_(c) values. In the case of superconducting wires whichuse the HTSC material of approximate composition Bi₂ Sr₂ Ca₂ Cu₃ O₁₀, itis common to utilise a combination of heat treatment and deformation inorder to grow well-aligned well-sintered grains thus allowing criticalcurrents at 77 K and zero field of up to 60,000 Amps/cm². This is thebasis of the powder-in-tube (PIT) technique wherein precursor powder isloaded into a silver tube which is drawn down in diameter then subjectedto a succession of cycles of partial reaction followed by rolling orpressing deformation to form high current-density tapes containingtextured Bi₂ Sr₂ Ca₂ Cu₃ O₁₀. A key element in achieving high texture inthis system is the fact that Bi₂ Sr₂ Ca₂ Cu₃ O₁₀ has an activedeformation slip system which allows quasi-plastic deformation ofindividual grains at room temperature.

However, other HTSC including TlBa₂ CaCu₂ O₇, Tl₀.5 Pb₀.5 Sr₂ CaCu₂ O₇,HgBa₂ CaCu₂ O₇ and other materials referred to generally as 1212, TlBa₂Ca₂ Cu₃ O₉, Tl₀.5 Pb₀.5 Sr₂ Ca₂ Cu₃ O₉, HgBa₂ Ca₂ Cu₃ O₉ and othermaterials referred to generally as 1223, while they have exceptionallygood flux pinning properties making these materials potentially far moreattractive for wire technology than Bi₂ Sr₂ Ca₂ Cu₃ O₁₀, do not possessan active slip system. As a consequence, simple deformation coupled withheat treatment fails to induce sufficient texturing to obtain goodcurrent transport properties. To the best of our knowledge up to thepresent no group has successfully made high current-density wires ortapes using the PIT technique with any HTSC other than the bismuthcuprates, Bi₂ Sr₂ CaCu₂ O₈ and Bi₂ Sr₂ Ca₂ Cu₃ O₁₀.

SUMMARY OF THE INVENTION

The invention comprises a method for forming a conductor elementcomprising a Tl or Hg-based high temperature superconductor (HTSC)material, comprising:

providing at least one first precursor material within an outer sheathfor said conductor element;

providing at least one second precursor material within said conductorsheath and separated from the first precursor material(s) by at leastone barrier layer between the first and second precursor materials; and

heating the conductor sheath containing the precursors to a temperatureat which the barrier layer(s) melt(s) to allow the precursor materialsto mix and react, or to a temperature at which one of the precursormaterial(s) diffuses through the barrier layer sufficiently to allow theprecursor materials to mix and react, to form the Tl or Hg-based HTSCmaterial within the outer conductor sheath.

Preferably said at least one barrier layer comprises at least one hollowtube within the conductor sheath and containing the first precursormaterial(s).

The second precursor material(s) may also be provided within theconductor sheath so as to surround the barrier tube(s) containing thefirst precursor material(s), or the second precursor material(s) mayalso be contained within at least one second hollow tube within theouter sheath adjacent the first barrier tube(s) containing the firstprecursor material(s). In the latter case the second hollow tube(s)forming a further barrier layer around the second precursor material(s)melts on heating of the conductor sheath containing the precursormaterials to the reaction temperature, or allows diffusion of theprecursor material(s) through the second barrier layer at the reactiontemperature.

In this specification "tube" is to be understood as including not onlyhollow bodies of circular cross-section but also of oval, square,rectangular, or other cross-sectional shapes and also such tubes inwhich the wall thickness of the tube varies around the cross-section ofthe tube.

DESCRIPTION OF PREFERRED FORMS

In one form of the method of the invention, the precursor materials orreactants required to synthesize the Tl or Hg-based HTSC material suchas any of the Tl-2212 or Tl-2223, or Tl or Hg-based 1212 or 1223, arephysically separated by, for example, loading one reactant as a powderor similar in one or more barrier tubes and the other reactants as apowder in one or more other barrier tubes, and placing these insideanother larger tube forming an outer conductor sheath. The HTSC is thenformed within the outer conductor sheath by heating the conductor sheathcontaining the precursor materials to the reaction temperature in anoxygen-containing atmosphere, such that one or more of the reactantseventually break through the separating metal walls to react with theremaining reactant(s) to produce a textured 2212, 2223, 1212 or 1223superconductor. Alternative to placing the one reactant or precursor orgroup of reactants or precursors in one barrier tube and anotherreactant or group of reactants or precursors in another barrier tube,the second reactant or precursor or mixture of second reactants orprecursors may be placed within the outer conductor sheath to surroundthe barrier tube containing the first reactant or group of reactants orprecursors.

In another form of the method of the invention the reactants may be inthe form of metal precursors, one or more of which are alloyed togetherto form a first precursor reactant, and some or all of the remainder ofwhich are alloyed together to form a second precursor reactant. Thereactants are then distributed in a wire such as a multifilamentary wirewithin a matrix of a barrier material, which forms a barrier layerbetween the reactants. The reactants are then oxidised at moderatetemperature in an oxygen-containing atmosphere, and then subsequentlyreacted at high temperature in an oxygen-containing atmosphere, suchthat one or more of the reactants eventually breaks through theseparating barrier matrix to react with the remaining reactant(s) toproduce the textured HTSC material. In another form, one or more of thereactants may be a metal precursor alloyed with a noble metal.

Preferably the barrier material or barrier tube(s) comprise an alloy ofa noble metal such as an alloy of silver, which may be an oxidedispersion strengthened alloy, or most preferably a pure noble metalsuch as silver. The walls of the barrier tube may be formed from thenoble metal or alloy, or the noble metal or alloy may be present as acoating or layer on or around another metal. One or more of thereactants may form a melt in combination with the noble metal whichallows the reactant to break through to the remaining reactants.Alternatively one or more of the reactants may be selected to have ahigh vapour pressure so as to allow significant in situ transport of thereactant(s) in the vapour phase through the barrier layer. Preferablyone or more of the remaining reactants forms a melt in combination withthe noble metal which enhances texturing of the cuprate superconductorat the interface with the noble metal upon reaction. As a furtherpreferment, the barrier of noble metal separating the reactants may besufficiently thick that one reactant does not break through to theothers until significant melting has occurred within some or all of theremaining reactants.

One of the precursor materials may comprise a member (which may itselfbe a HTSC), such as a lower member, of the homologous series ofcompounds of which the HTSC is also a member, as is further described.

Preferably before reacting, the conductor sheath containing theprecursor material(s) within one or more barrier tubes or in a noblemetal matrix, is reduced to a smaller diameter by drawing, extruding,rolling, pressing or other deformation, to densify the reactants and toincrease the surface to volume ration of the reactants. Alternativelythe metal tube(s) containing the precursor material(s) may be reduced toa smaller diameter prior to loading into the outer conductor sheath.

The reaction stage is carried out at a temperature and oxygen pressurewhich produces the HTSC from the precursor(s), but which preferably alsofavours the growth of highly platey grains of the HTSC during thereaction. For example, for Tl-1212 the reaction may be carried out at atemperature in the range 750° C. to 870° C. and most preferably 750° C.to 850° C. and an oxygen partial pressure less than 1 bar and preferablyless than 0.2 bar, for Tl-1223 the reaction may be carried out at atemperature in the range 800° C. to 935° C. and most preferably 870° C.to 925° C. and an oxygen partial pressure of 1 bar or less.

After reacting the conductor may be again drawn, rolled, pressed orsubjected to other mechanical deformation to reduce any voids left bymigration of the precursor material(s) during the reaction. During thereaction gaseous or mechanical pressure may be applied to collapse suchvoids and to assist in sintering the HTSC grains in the conductorsheath.

The process of the invention may be carried out to produce conductorscomprising any HTSC including small variations in stoichiometry from thenominal compositions or any substituted form generally recognised in theart as being of Tl-2212, Tl-2223, Tl-1201, Tl-1212, Tl-1223, Hg-1201,Hg-1212, or Hg-1223 composition.

By 1212 is meant the nominal compositions TlSr₂ CaCu₂ O₇ or HgBa₂ CaCu₂O₆₊δ where, for example, Tl is optionally partially substituted by Pband/or Bi, Hg is optionally partially substituted by Tl, Pb or Bi, Sr isoptionally partially substituted by La or Ba, Ba is optionally partiallysubstituted by Sr or La, and Ca is optionally partially substituted byR. These include Tl₀.5 Pb₀.5 Sr_(2-w) Ba_(w) CaCu₂ O₇, Tl₀.5 Pb₀.5Sr_(2-x) La_(x) CaCu₂ O₇, Tl₀.5 Pb₀.5 Sr₂ Ca_(1-y) R_(y) Cu₂ O₇,Tl_(1-y) Bi_(y) Sr_(2-u-v) La_(u) Ba_(v) CaCu₂ O₇, and Tl_(1-y) Bi_(y)Sr_(2-v) Ba_(v) Ca_(1-u) R_(u) Cu₂ O₇ where 0≦x,w≦2, 0≦u,v,y≦1 and R isY or any of the lanthanide rare earth elements, or more generally(Tl,Pb,Bi)₁ (Sr,Ba,La)₂ (Ca,Tl,R)₂ Cu₂ O₇. Also, La may be partially orcompletely substituted by Nd.

By 1223 is meant the nominal compositions TlSr₂ Ca₂ Cu₃ O₉ or HgBa₂ Ca₂Cu₃ O₈₊δ where Tl is optionally partially substituted by Pb and/or Bi,Hg is optionally partially substituted by Tl, Pb or Bi, Sr is optionallypartially substituted by Ca or Ba and Ba is optionally partiallysubstituted by Sr or La. These include Tl₀.5 Pb₀.5 Sr_(2-x) Ca₂ Ca_(x)Ca₂ Cu₃ O₉, Tl₀.5 Pb₀.5 Sr_(2-w-x) Ba_(w) Ca_(x) Ca₂ Cu₃ O₉, andTl_(1-y) Bi_(y) Sr_(2-w) Ba_(w) Ca_(2-u) R_(u) Cu₃ O₉ where 0≦x,y,u,w≦1or more generally (Tl,Pb,Bi)₁ (Sr,Ba,La,Ca)₂ (Ca,Tl,R)₂ Cu₃ O₉.Preferred compositions for the thallium 1223 cuprate superconductorsinclude Tl₀.9 Bi₀.1 BaSr₁.2 Ca₁.8 Cu₃ O₉ and Tl₀.6 Pb₀.4 Ba₀.4 Sr₁.6Ca₁.9 Cu₃ O₉.

In all of these compounds, as is known in the art, there may exist somedegree of oxygen non-stoichiometry such that O₅, O₇ and O₉ may beinterpreted as O₅₊δ, O₇₊δ and O₉₊δ, in which -0.3≦δ≦+0.3 for example.There may also exist some degree of cation non-stoichiometry (forexample Ca may be deficient by up to 0.2 mole fraction or Ba+Sr may bein excess) but the compounds will be generally recognised in the art bythe above-noted indices.

Where the HTSC is Tl-1212 compound or 1223 compound as described abovethe first precursor material may be Tl₂ O₃ and the second precursormaterial may be the thallium-free remnant oxide material (designated0212 or 0223 respectively) such as to provide the 1212 or 1223 compoundon full reaction with the Tl₂ O₃.

Alternatively the first precursor material may be Tl-2212 or 2223 andthe second precursor material may have the composition designated 0212or 0223 respectively such that upon reaction 2212 and 0212->1212 or2223+0223->2223. An exemplary reaction would be Tl₂ Ba₂ Ca₂ Cu₃ O₁₀ +Ba₂Ca₂ Cu₃ O_(x) ->TlBa₂ Ca₂ Cu₃ O₉.

DESCRIPTION OF DRAWINGS

In the accompanying figures:

FIGS. 1a-1i show examples of wire architectures for disposing filamentscontaining separated reactants according to the process of theinvention;

FIGS. 2a to 2f show further examples of wire architectures for disposingfilaments containing separated reactants according to the process of theinvention;

FIGS. 3a and 3b show formation of non-circular cross-section filaments;

FIG. 4a shows one particular multifilamentary wire architecture and FIG.4b shows the architecture of FIG. 4a after deforming into a tape;

FIG. 5 shows a scanning electron micrograph of the cross section ofseparated-reactants tape processed according to example 1 showing theformation of textured plates of thallium-1223 at the oxide-silverinterface--the black cavity is the remnant of the empty inner wire whichcontained the Tl₂ O₃ source;

FIG. 6 shows a scanning electron micrograph of the cross section ofseparated-reactants tape processed according to example 3 showing theformation of textured plates of thallium-1223 at the oxide-silverinterface; and

FIG. 7 is a graph of the critical current for the tape of Example 5below plotted in amps against the applied magnetic field parallel andperpendicular to the normal vector from the plane of the tape.

DESCRIPTION OF PREFERRED FORMS OF WIRE ARCHITECTURES

FIGS. 1a to 1iand 2a to 2f show various preferred forms of architecturesfor conductors formed by the method of the invention.

In FIG. 1a a barrier tube 10 contains a thallium core 12 and the barriertube 10 is in turn surrounded by the non-thallium precursor materials 14which are in turn contained with an outer sheath 16. In FIG. 1b a seriesof barrier tubes 10 are arranged within an outer sheath 16 eachcontaining a thallium or mercury core 12 and are each surrounded bynon-thallium or mercury precursor materials 14 which may in turn becontained with an outer tube 15, within the outer sheath. In FIG. 1cthree barrier tubes 10 containing thallium or mercury precursors 12 arearranged with other barrier tubes 15 containing non-thallium or mercuryprecursors 14, as shown the--number of tubes in such a configuration isvariable as desired. In FIG. 1d a central core 12 of thallium or mercuryprecursor within a barrier tube 10 is surrounded by barrier tubes 15containing non-thallium or mercury precursors 14. Such conductors orfilaments may in turn be contained as sub-units within a high filamentcount multifilamentary wire as shown in FIG. 1e, which may in turn formthe sub-units of an again higher filament count multifilamentary wire asshown in FIG. 1f. FIGS. 1g to 1i show yet further arrangements ofthallium or mercury and non-thallium or mercury precursors within anouter sheath.

In FIG. 2a the non-thallium or mercury precursors 214 are containedwithin a barrier tube 215 which is a wide tape-like tube and thethallium or mercury precursors 212 are contained within narrowerdiameter tape-like tubes 210 on one side. In FIG. 2b thallium or mercurycontaining tapes or tubes 210 are arranged on both sides of thenon-thallium or mercury containing tape-like tube 215, and in FIG. 2calternating layers of thallium or mercury containing tubes 210 andnon-thallium or mercury containing tubes 215 are stacked together asshown. In FIG. 2d, a wider tape-like tube 215 containing non-thallium ormercury precursor material 214 is surrounded by a number of thallium ormercury containing tubes 210 of smaller dimensions on either side. FIGS.2e and 2f show arrangements of thallium and mercury and non-thallium andmercury containing tubes (210, 215), all of which are arranged in thesame plane.

FIGS. 3a and 3b show how tape like tubes containing precursor materials,or tubes of non-circular cross section, may be formed. Hollow tubes 310,315 of circular cross-section containing thallium or mercury precursormaterial 312 (FIG. 3a) or non-thallium or mercury precursor material 314(FIG. 3b) are deformed by pressing or rolling or similar to non-circularcross-sectional shapes 310', 315'. The core precursor materials may bepacked into the circular tubes as powders prior to deformation.Alternatively, circular or non-circular tubes may be formed by extrusionof the barrier material to a circular or non-circular shape. Powderedprecursor material may be packed into the barrier tube at the point offormation of the barrier tube by extrusion or similar so that thebarrier tube is effectively extruded around the powdered core, oralternatively a core of thallium or mercury metal may be extruded intoand together with a surrounding barrier layer such as a noble metal.

FIG. 4a shows how thallium or mercury and non-thallium or mercurycontaining tubes such as the configuration of FIG. 2c, may be packedtogether in an outer sheath as shown in FIG. 4a. The outer sheath 416may be extruded or pressed or rolled to the tape configuration of FIG.4b, for example, while maintaining the fundamental architecture of thetube or tape.

The precursor materials may be loaded into the tubes by packing powdersor spin coating powders in fluid suspension and evaporation of the fluidcarrier or by other suitable techniques.

The wire architectures described and shown in the drawings are describedby way of example only.

EXAMPLES

The invention is further illustrated by the following examples:

Example 1

Separated reactants were reacted to form the 1223 superconductor Tl₀.9Bi₀.1 BaSr₁.2 Ca₁.8 Cu₃ O_(x) using the powder-in-tube technique. Tl₂ O₃powder was loaded into a silver tube and drawn down to about 1.8 mmdiameter. This wire was inserted into a silver tube and oxide precursorwith stoichiometry Bi₀.1 BaSr₁.2 Ca₁.8 Cu₃ O_(x) was packed around thecentral Tl₂ O₃ wire to make a composite wire. The precursor comprised amixture of BaCu₂ O₃ and the remnant precursor to make up thestoichiometric composition Bi₀.1 BaSr₁.2 Ca₁.8 Cu₃ O_(x). The BaCu₂ O₃was adopted because it forms a melt at about 890° C. in oxygen. Theremnant precursor was made by mixing a stoichiometric mixture of thenitrates of Sr and Ca and oxides of Cu and Bi, decomposing the mixtureby heating slowly to 750° C. in air and milling. The composite wire wasdrawn down to 0.5 mm diameter and rolled to a thickness of 0.2 mm, thenreacted by heating to 900° C.

The critical current was determined, then the tape was sectioned forx-ray diffraction (XRD) and scanning electron microscopy (SEM)characterisation. FIG. 1 shows an SEM micrograph of a longitudinalsection of the tape. Thallium oxide has broken through the wall of thecentral wire and reacted with the oxide precursor to form the desired1223 composition. It can be seen that the 1223 has adopted significantlevels of texture along the silver walls.

Further examples were explored in which the thallium source wire wasdrawn to different diameters, the composite wire was drawn to differentdiameters and rolled to different thicknesses in order to vary andcontrol the silver wall thickness separating the reactants. Where thepre-reaction deformation was sufficiently large that the thallium sourcematerial ruptured through to the oxide reactant 1223 the resulting oxidewas amorphous and there occurred little or no texturing of the fractionof 1223 that did form.

Example 2

The process of example 1 was repeated using a stoichiometric compositionTl₀.6 Pb₀.4 Ba₀.4 Sr₁.6 Ca₁.9 Cu₃ O_(x). Similar texture was achievedalong the silver walls and notably the composition of the reacted oxidewas more homogeneous than was observed for the composition of example 1.Further experiments carried out on the same composition and heatingschedule but where the reactants were not separated but homogeneouslymixed showed that no such texturing occurred at the silver walls.

Example 3

The process of example 1 was repeated but using a different architecturefor the separated reactant wires similar to that shown in FIG. 2d. The1.3 mm diameter Tl₂ O₃ wire was surrounded with 7 wires drawn to about1.2 mm diameter containing the non-thallium precursor of stoichiometriccomposition Bi₀.1 BaSr₁.2 Ca₁.8 Cu₃ O_(x). The bundle of 8 wires wasloaded into a silver tube then drawn, rolled and reacted as described inexample 1. During reaction the thallium broke through to the closestupper and lower filaments where a high degree of texture of thethus-formed 1223 was observed at the silver walls as shown in FIG. 3.Critical current densities of up to 24,000 amps/cm² were observed forthese filaments.

Example 4

High-filament-count multifilamentary wires where a group of filamentwires such as those of example 3 are cut into lengths, rebundled, drawnand rebundled multiple times will show similar texturing effects onreaction especially where metal precursors are used and the uniformityof the separating wall thickness is better controlled. Other examples offilament architectures are shown in FIG. 2. These and other suchfilaments may be used as the single composite wire or may be thesubunits of a high-filament-count multifilamentary wire.

Example 5

A silver tube was loaded with Tl₂ O₃ powder wire, drawn to down 1.8 mmdiameter and then turned down to a 1.65 mm diameter. This was placedinside a 6 mm outer diameter, 4 mm inner diameter silver tube and theannular space between the tubes was packed with precursor oxide powderof composition Pb₀₄ Ba₀.4 Sr₁.6 Ca₁.9 Cu₃ _(x), the relative proportionsof precursor and Tl₂ O₃ powder being designed to yield a final overallcomposition of Tl₀.6 Pb₀.4 Ba₀.4 Sr₁.6 Ca₁.9 Cu₃ O_(x). This compositewas then drawn to 0.5 mm and rolled to a 0.2 mm thickness tape. Sectionsof the wire tape were then cut and crimped closed at the ends and thenheat treated by ramping from 690° C. to 920° C. at 1° C./min, held therefor 60 min then ramped down to 840° C. at 0.5° C./min then withdrawnfrom the furnace. Scanning electron micrographs showed good areas ofgrain alignment away from where the thallium oxide had ruptured through,with very good alignment at the silver/superconductor interface. FIG. 7shows the critical current for this tape plotted in amps against theapplied magnetic field parallel and perpendicular to the normal vectorfrom the plane of the tape. There is a marked anisotropy in criticalcurrent as high as 2x in the mid-field region. It is believed that thisanisotropy results from the grain alignment achieved in the core of thewire tape.

The foregoing describes the invention including a preferred formthereof. Alterations and modifications as will be obvious to thoseskilled in the art are intended to be incorporated within the scopehereof as defined in the claims.

We claim:
 1. A method for forming a conductor element comprising a Tl orHg-based high temperature superconductor (HTSC) material,comprising:providing at least one first precursor material within anouter sheath for said conductor element; providing at least one secondprecursor material within said conductor sheath and separated from thefirst precursor material(s) by at least one barrier layer between thefirst and second precursor materials; and heating the conductor sheathcontaining the precursors to a temperature at which the barrier layer(s)melt(s) to allow the precursor materials to mix and react, or to atemperature at which one of the precursor material(s) diffuses throughthe barrier layer sufficiently to allow the precursor materials to mixand react, to form the Tl or Hg-HTSC material within the outer conductorsheath.
 2. A method according to claim 1, wherein said at least onebarrier layer comprises at least one hollow tube within the conductorsheath and containing the first precursor material(s).
 3. A methodaccording to claim 2, including providing the second precursormaterial(s) within the conductor sheath so as to surround the barriertube(s) containing the first precursor material(s).
 4. A methodaccording to claim 2, including providing the second precursormaterial(s) within at least one second hollow tube within the outersheath adjacent the first barrier tube(s) containing the first precursormaterial(s), said second hollow tube(s) forming a further barrier layer,around the second precursor material(s), which melts on heating of theconductor sheath containing the precursor materials to said reactiontemperature, or allows diffusion of the precursor material(s) throughsaid second barrier layer at said reaction temperature.
 5. A methodaccording to claim 3, wherein the first precursor material(s) are loadedin said barrier tube(s) which is/are then reduced to a smaller diameterby drawing or extruding the hollow tube(s) containing the precursormaterial(s) prior to loading the barrier tube(s) into the outerconductor sheath.
 6. A method according to claim 4, wherein the secondprecursor material(s) are loaded in said barrier tube(s) which is/arethen reduced to a smaller diameter by drawing or extruding prior toloading the barrier tube(s) into the outer conductor sheath.
 7. A methodaccording to claim 5, wherein the second precursor material(s) areloaded in said barrier tube(s) which is/are then reduced to a smallerdiameter by drawing or extruding prior to loading the barrier tube(s)into the outer conductor sheath.
 8. A method according to claim 1,wherein said barrier layer is formed from a Noble metal.
 9. A methodaccording to claim 2, wherein said barrier tube(s) are formed from aNoble metal.
 10. A method according to claim 3, wherein said barriertube(s) are formed from a Noble metal.
 11. A method according to claim4, wherein said barrier tube(s) are formed from a Noble metal.
 12. Amethod according to claim 1, wherein said barrier layer is formed fromsilver metal.
 13. A method according to claim 2, wherein said barriertube(s) are formed from silver metal.
 14. A method according to claim 3,wherein said barrier tube(s) are formed from silver metal.
 15. A methodaccording to claim 4, wherein said barrier tube(s) are formed fromsilver metal.
 16. A method according to claim 2, wherein said barriertube(s) are formed from an oxide dispersion strengthened alloy.
 17. Amethod according to claim 1, wherein one or both of the first and secondprecursor materials each comprise a metal oxide or an alloy comprisingmetal oxide components.
 18. A method according to claim 1, wherein theHTSC is of nominal composition TlSr₂ CaCu₂ O₇₋δ (1212) where-0.3≦δ≦+0.3, Tl may be partially substituted by Pb and/or Bi, Sr may bepartially substituted by La and/or Ba, and Ca may be partiallysubstituted by Y or any of the lanthanide rare earth elements.
 19. Amethod according to claim 18, wherein the HTSC is selected from thegroup consisting of Tl₀.5 Pb₀.5 Sr_(2-w) Ba_(w) CaCu₂ O₇₋δ, Tl₀.5 Pb₀.5Sr_(2-x) La_(x) CaCu₂ O₇₋δ, or Tl₀.5 Pb₀.5 Sr₂ Ca_(1-y) R_(y) Cu₂ O₇₋δwhere 0≦x,w≦, 0≦y≦1, -0.3≦δ≦+0.3, R is Y or any of the lanthanide rareearth elements, and La may be partially or completely substituted by Nd.20. A method according to claim 1, wherein the HTSC is of nominalcomposition TlSr₂ Ca₂ Cu₃ O₉₋δ (1223) where -0.3≦δ≦+0.3, Tl may bepartially substituted by Pb or Bi and Sr may be partially or fullysubstituted by Ba.
 21. A method according to claim 20, wherein the HTSCis selected from the group consisting of Tl₀.5 Pb₀.5 Sr_(2-x) Ca_(x) Ca₂Cu₃ O₉₋δ, Tl₀.5 Pb₀.5 Sr_(2-w-x) Ba_(w) Ca_(x) Ca₂ Cu₃ O₉₋δ or Tl_(1-x)Bi_(x) Sr_(2-w) Ba_(w) Ca₂ Cu₃ O₉₋δ where 0≦x,w≦1 and -0.3≦δ≦+0.3.
 22. Amethod according to claim 1, wherein the first precursor material is Tl₂Ba₂ CaCu₂ O₈ and the second precursor material is of composition Ba₂CaCu₂ O_(x) where Tl may be partially substituted by Pb or Bi, Ba may bepartially substituted by Sr or La and Ca may be partially substituted byR where R is Y, or a lanthanide rare earth.
 23. A method according toclaim 1, wherein the first precursor material is Tl₂ Ba₂ Ca₂ Cu₃ O₁₀ andthe second precursor material is of composition Ba₂ Ca₂ Cu₃ O_(x) whereTl may be partially substituted by Pb or Bi, and Ba may be partiallysubstituted by Sr.
 24. A conductor formed by the method of claim
 1. 25.A conductor formed by the method of claim 4.